Voltage tunable laminated dielectric materials for microwave applications

ABSTRACT

A tunable dielectric structure includes a first layer of dielectric material, a second layer of dielectric material positioned adjacent to the first layer of dielectric material, with the second layer of dielectric material having a dielectric constant that is less than the dielectric constant of the first layer of dielectric material, and electrodes for applying a controllable voltage across the first dielectric material, thereby controlling a dielectric constant of the first dielectric material, wherein at least one of the electrodes is positioned between the first and second layers of dielectric material. The dielectric materials can be formed in various shapes and assembled in various orientations with respect to each other. The tunable dielectric structure is used in various devices including a coaxial cables, cavity antenna, microstrip lines, coplanar lines, and waveguides.

CROSS REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 60/104,503, filed Oct. 16, 1998.

BACKGROUND OF INVENTION

The present invention relates generally to electronic materials formicrowave applications, and more particularly to such materialsincluding ferroelectric materials having a tunable dielectric constant.

Tunable ferroelectric materials are the materials whose permittivity(more commonly called dielectric constant) can be varied by varying thestrength of an electric field to which the materials are subjected orimmersed. Even though these materials work in their paraelectric phaseabove Curie temperature, they are conveniently called “ferroelectric”because they exhibit spontaneous polarization at temperatures below theCurie temperature. Typical tunable ferroelectric materials arebarium-strontium titanate (BST) or BST composites. Examples of suchmaterials can be found in U.S. Pat. Nos. 5,312,790, 5,427,988, 5,486,491and 5,643,429. These materials, especially BSTO-MgO composites, show lowdielectric loss and high tunability. Tunability is defined as thefractional change in the dielectric constant with applied voltage. Theseunique properties make these materials suitable for microwaveapplications such as phase shifter, tunable filters, tunable resonators,and delay lines.

U.S. Pat. No. 5,830,591 discloses a multi-layer ferroelectric compositewaveguide in which the effective dielectric constant of the waveguidecan be reduced while maintaining tunability. The waveguide isconstructed of high and low dielectric constant layers. The multi-layerwaveguide is comprised of bias plates that are perpendicular to thelaminate direction to maintain tunability in the structure. Thestructure disclosed in U.S. Pat. No. 5,830,591 is only suitable forwaveguide applications. Since high dielectric fields, for example about10 V/μm, are necessary to tune tunable material, especially in waveguideapplications, the distance between bias electrodes should be kept small.With the bias plate arrangement of U.S. Pat. No. 5,830,591, multiplelayers would be needed along the direction of that bias plates as wellas in the direction of the laminated dielectric material stack. Thismakes fabrication of such devices complex.

U.S. Pat. No. 5,729,239 discloses a device for scanning in a scanningplane that includes a periodic array of conductive plates disposed alongthe scanning axis, adjacent plates being disposed about half awavelength apart. The device has a periodic array of slabs disposedalong the scanning axis, each slab comprising ferroelectric material,being disposed between a pair of adjacent conductive plates of theperiodic array of conductive plates, with adjacent slabs being separatedby one of the conductive plates. Each of the slabs has a receiving faceand a radiating face substantially parallel to each other. Each of theslabs transmits an electromagnetic signal from the receiving face to theradiating face. Input transmission means feed an input electromagneticsignal to the periodic array of slabs in a propagation direction so thatthe input electromagnetic signal is incident on the receiving faces ofeach of the slabs and so that the electrical component of the inputelectromagnetic signal received at each receiving face has a componentparallel to the scanning axis. Output transmission means transmit anoutput signal from the periodic array of slabs responsive to theelectromagnetic signal transmitted from each receiving face in thecorresponding slab. The device also includes a plurality of means forselectively applying a voltage across each of the pairs of conductiveplates disposed about a slab so as to selectively control the phase ofthe electromagnetic signal received at each of the radiating faceshaving been transmitted from the receiving face in the correspondingslab.

U.S. Pat. No. 5,729,239 discloses the use of barium strontium titanate(BSTO), or composites thereof, or other ceramics as the ferroelectricmaterial. BSTO-MgO has also been proposed by others for use as a tunableferroelectric material. However, the materials in the BSTO-MgO systemgenerally have dielectric constants of over 100. The high dielectricconstant is not suitable for some microwave applications such as patchantennas, which lower the antennas' efficiencies. High dielectricconstant materials also cause low characteristic impedance (<10Ω) inmicrostrip, coplanar, and other planar structure transmission lines,which strongly limits the application of high dielectric constantmaterials. Low dielectric constant materials (for example, withdielectric constants less than 40) with low loss and high tunability aredesired for patch antennas and other microwave applications.

It would be desirable to construct a ferroelectric structure having arelatively low overall dielectric constant that takes advantage of thehigh tunability and low loss characteristics of materials such as BSTcomposites, having high dielectric constants. It is further desired toconstruct such structures for use in various microwave devices such asmicrostrips, coplanar or other planar microwave transmission lines,coaxial cable, or waveguides

SUMMARY OF THE INVENTION

This invention provides a tunable dielectric structure including a firstlayer of dielectric material, and a second layer of dielectric materialpositioned adjacent to the first layer of dielectric material, with thesecond layer of dielectric material having a dielectric constant that isless than the dielectric constant of the first layer of dielectricmaterial. The structure further includes electrodes for applying acontrollable voltage across the first dielectric material, therebycontrolling a dielectric constant of the first dielectric material,wherein at least one of the electrodes is positioned between the firstand second layers of dielectric material.

The dielectric materials can be formed in various shapes and assembledin various orientations with respect to each other. Laminated structuresof such dielectric materials can serve as substrates for microstrips,coplanar or other planar microwave transmission lines, as well asdielectric media for coaxial cable or waveguides.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the invention can be gained from the followingdescription of the preferred embodiments when read in conjunction withthe accompanying drawings in which:

FIG. 1 is isometric view of a dielectric structure of the laminatedmaterial constructed in accordance with a preferred embodiment of theinvention;

FIG. 2 is schematic representation of a laminated structure inaccordance with the invention;

FIG. 3 is a schematic of the equivalent electric circuit of a laminatedstructure in accordance with the invention;

FIG. 4 is an end view of a coaxial line that includes a dielectricstructure of the laminated material constructed in accordance with theinvention;

FIG. 5 is a cross sectional view of the structure of FIG. 4, taken alongline 5—5;

FIG. 6 is an end view of another alternative embodiment of the inventionfor antenna applications;

FIG. 7 is a cross sectional view of the structure of FIG. 6, taken alongline 7—7;

FIG. 8 is isometric view of a microstrip line that includes a dielectricstructure of the laminated material constructed in accordance with theinvention;

FIG. 9 is isometric view of a coplanar line that includes a dielectricstructure of the laminated material constructed in accordance with theinvention; and

FIG. 10 is an isometric view of a waveguide that includes a dielectricstructure of laminated material constructed in accordance with theinvention..

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, FIG. 1 is an isometric view of an electronicdevice having a dielectric structure of laminated materials constructedin accordance with a preferred embodiment of the invention. The tunabledielectric device 10 comprises a multilayered structure of dielectricmaterials 12, 14, 16, 18 including two or more materials havingdifferent dielectric constants, and being laminated together to tailorthe overall dielectric constant and tunability. One or more materials(e.g. 12 and 16) in the laminated structure are tunable dielectricmaterials usually with a high dielectric constant, low losses, and hightunability. For the purposes of this description, a high dielectricconstant is greater than about 100, low loss materials have losstangents (tan δ) of less than about 0.01, and tunability of greater thanabout 15% at 2 V/μm. The high dielectric constant, low loss and hightunability materials may be Ba_(1−x)Sr_(x)TiO₃ (BSTO), where x can varybetween zero and one, and composites thereof that exhibit such lowlosses and high tunability. Examples of such composites include, but arenot limited to: BSTO-MgO, BSTO-MgAl₂O₄, BSTO-CaTiO₃, BSTO-MgTiO₃,BSTO-MgSrZrTiO₆, or any combination thereof. The other materials (e.g.14 and 18) in the laminated structure may be tunable or non-tunabledielectric materials such as Ba_(1−x)Sr_(x)TiO₃-based materials,alumina, Mica, and air. Since air is the lowest dielectric material andthe lowest loss material, it is particularly useful for certainapplications. If air is used as the non-tunable dielectric in thestructures of this invention, the tunable materials would be mountedwith air gaps between tunable layers. The resultant overall dielectricconstant, tunability and other properties of the laminated material isdependent on the relative properties and thickness of each of thelayered materials. Therefore, these properties can be tailored byvarying the number of layers of dielectric materials with certaindielectric constant characteristics, and varying the thickness of thelayers.

The laminate material of the present invention appears as a uniformmaterial to a radio frequency signal that is applied to the structure.While the structures of the invention are not limited to any particulardimensions, the thickness of the layers should be such that thisapparent uniformity is achieved. In the preferred embodiments, thatthickness of the dielectric layers is smaller that one tenth of thewavelength of the radio frequency signal to be used with the device.

In the embodiment of FIG. 1, each of the dielectric materials is in theform of a rectangular slab. Each slab has an input end 20 for receivinga radio frequency signal (RF_(IN)) and an output end 22 for deliveringthe signal (RF_(OUT)). In general, the laminated structure of FIG. 1 canserve as a tunable dielectric media for microwave transmission. Themeans for inputting and outputting a radio frequency signal from thestructure will depend upon the application in which it is used.Electrodes 24, 26, 28, 30 and 32, in the form of sheets of conductivematerial are located at each end of the stack and between each of thetunable dielectric materials. The electrodes are positioned adjacent toopposite faces of at least each slab that is comprised of tunableferroelectric material. With this structure, at least some of theelectrodes are positioned within the laminate stack and lie in planesparallel to the direction of propagation of the RF signal and parallelto opposite faces of the slabs of tunable material. For those dielectricmaterial slabs that have a voltage controlled dielectric constant, acontrollable DC voltage source 34 is electrically connected to theelectrodes on opposite sides of the slab. In FIG. 1, only onecontrollable DC voltage source is shown, but it must be understood thatadditional voltage sources may be used to control the dielectricconstant of the several slabs, or the same DC voltage source may beconnected to multiple slabs of dielectric material. In the preferredembodiments, layers of the same tunable dielectric material would besubject to the same bias voltage. In addition, the polarity of theapplied voltage can be changed without affecting performance of thedevice. A coordinate system is illustrated in FIG. 1 such that the slabslie in planes parallel to the y-z plane, and are stacked in the xdirection. The radio frequency signal propagates in the y directionthrough the device.

FIG. 2 is a schematic representation of a laminated structure inaccordance with the invention. In the embodiment depicted in FIG. 2, aplurality of slabs 36, 38, 40 and 42 of dielectric material are shown tohave dielectric constants of _(∈) 1, _(∈) 2, _(∈) 3, through _(∈)n, andthickness t1, t2, t3, through tn, respectively. FIG. 2 shows a structurethat includes two assemblies 44 and 46, each having the same arrangementof dielectric materials. A plurality of electrodes, for example 48, 50and 52 are positioned between the dielectric slabs and are connected toone or more controllable DC voltage sources. In FIG. 2, one controllablevoltage source 54 is shown for clarity. However, as discussed abovemultiple sources, and/or multiple connections to a single source may beused in operational devices. This figure illustrates that a completedevice can be comprised of multiple subassemblies, each having the sameor a similar arrangement of dielectric materials. Coordinates x, y and zin FIG. 2 correspond to coordinates x, y and z in FIG. 1.

FIG. 3 is a schematic of the equivalent electric circuit of a laminatedstructure in accordance with the invention. In FIG. 3, at least selectedones of the various values of capacitance C₁, C₂, C₃, through C_(n), canbe changed by varying the control voltages applied to the dielectricslabs that contain tunable ferroelectric material. The overallcapacitance of the laminated structure is the sum of the capacitance ofthe individual slabs.

FIG. 4 is an end view of an alternative embodiment of the invention fortunable coaxial cable applications in which the dielectric material isarranged in concentric cylinders 56, 58, 60, and 62. Here again, some ofthe layers of dielectric material can be tunable material havingrelatively high dielectric constants, low losses and high tunability,while the other layers can be tunable or non-tunable material.Concentric cylindrical electrodes 64, 66, 68 and 70 are positionedbetween the dielectric materials so that a bias voltage can be appliedto the control the dielectric constants of the dielectric cylinders thatcontain tunable ferroelectric material. A metallic center conductor 72,and a cylindrical metallic ground 74 are provided to carry the RF signalthrough the cable.

FIG. 5 is a cross sectional view of the structure of FIG. 4, taken alongline 5—5. One of the controllable DC voltage sources is shown to beconnected to electrodes 68 and 70. Additional controllable voltagesources (not shown) would be used for applying bias voltages to otherelectrodes that lie adjacent to the internal and external surfaces ofthe tunable layers. The direction of propagation of a radio frequencysignal through the structure is illustrated by arrow 76. The cylindricalelectrodes are positioned around an axis that lies parallel to thedirection of propagation of the radio frequency signal through thedevice. In FIG. 5, items 56, 58, 60, 62, 64, 72 and 74 identify thatsame structures as identified by those item numbers in FIG. 4.

FIG. 6 is an end view of another embodiment of the invention in the formof a tunable cavity antenna that includes a plurality of rectangularslabs of dielectric material, a representative sample of which arenumbered as items 82, 84, 86 and 88. Each of the slabs has a pair ofelectrodes, as illustrated by items 90 and 92, on opposite sides thereoffor the application of a bias voltage. As shown in the figure, the slabsare arranged such that certain slabs lie in planes that areperpendicular to the planes occupied by certain other slabs. Thisinvention provides a tunable cavity for a cavity antenna by placing alaminated, tunable material, with a specific dielectric constant, intothe cavity. The open spaces in the cavity of FIG. 6 can be filled withair or a non-tunable dielectric material.

FIG. 7 is a cross sectional view of the structure of FIG. 6, taken alongline 7—7. In FIG. 7, items 82, 84, 86, 88, 90, and 92 identify that samestructures as identified by those item numbers in FIG. 6. A controllablevoltage source 94 is shown to supply a controllable bias voltage toelectrodes 90 and 92, thereby controlling the dielectric constant of thedielectric material 82. While only one controllable voltage source isshown, it will be appreciated by those skilled in the art thatadditional controllable voltage sources, or alternative connects to asingle source, would be used to practice the invention. Arrow 96illustrates the direction of propagation of a radio frequency signalthrough the device. In the structure of FIGS. 6 and 7, selected ones ofthe dielectric slabs can contain material having a relatively highdielectric constant, low losses, and high tunability. The other slabs ofdielectric material can be tunable or non-tunable materials.

FIG. 8 is isometric view of a microstrip line that includes a dielectricstructure of the laminated material constructed in accordance with theinvention. In this embodiment, a laminated structure 10′ similar to thatof FIG. 1 is constructed of a plurality of slabs of dielectric material,as illustrated by items 98, 100, 102 and 104. Here again, electrodes arepositioned on opposite sides of the slabs, as illustrated by electrodes106, 108, 110, and 112. The laminated structure 10′ is mounted on aground plane 114 such that the slabs are positioned generallyperpendicular to the ground plane. A microstrip 116 is mounted on a sideof the laminated structure opposite the ground plane.

FIG. 9 is isometric view of a coplanar line that includes a dielectricstructure of the laminated material constructed in accordance with theinvention. In this embodiment, a laminated structure 10″ similar to thatof FIG. 1 is constructed of a plurality of slabs of dielectric material,as illustrated by items 118, 120, 122, and 124. Here again, electrodesare positioned on opposite sides of the slabs, as illustrated byelectrodes 126, 128, 130, and 132. A center strip 134 and two groundplanes 136 and 138 are mounted on one side of the laminated structure.The ground planes are positioned on opposite sides of the center strip.The RF signal is transmitted through the center strip and the adjacentground planes.

FIG. 10 is an isometric view of a waveguide that includes a dielectricstructure of laminated materials constructed in accordance with theinvention. In this embodiment, a laminated structure 10′″ similar tothat of FIG. 1 is constructed of a plurality of slabs of dielectricmaterial, as illustrated by 140, 142, 144, and 146. Here again, theelectrodes are positioned on opposite sides of the tunable slabs, asillustrated by electrodes 148, 150, 152 and 154. The laminated materialis filled into the waveguide 156 in a manner similar to that used forknown dielectric loaded waveguides. The RF signal is then input andoutput in accordance with known techniques.

The laminated dielectric material structure of the present invention canprovide certain overall dielectric constant(s) and tunability bylaminating high dielectric constant, high tunability material(s) withlow dielectric constant tunable or non-tunable material(s) withoutsubstantial lowering of their tunability, or degradation of dielectricloss. For the purposes of this invention, high dielectric materials havea dielectric constant greater than about 100, and low dielectricmaterials have a dielectric constant lower than about 30.

FIG. 1 illustrates the concept of the present invention, which consistsof n (n≧2) layers of different materials with dielectric constants∈_(n), and thickness t_(n). Since the equivalent circuit is that ofparallel capacitors, the resultant dielectric constant ∈_(e) of thelaminated material is expressed as following: $\begin{matrix}{{ɛ_{e}^{0} = {\sum\limits_{i = 1}^{n}\quad {\frac{t_{i}}{T}ɛ_{i}^{0}}}};} & (1) \\\left( {T = {\sum\limits_{i = 1}^{n}\quad t_{i}}} \right) & \quad\end{matrix}$

here ∈_(e) ⁰ is the resultant dielectric constant of the laminatedmaterials at no dc bias, t_(i) is the thickness of the ith layer, T isthe total thickness of the laminated materials, ∈_(i) ⁰ is thedielectric constant of the ith layer at no dc bias.

Under dc bias conditions: $\begin{matrix}{{ɛ_{e}^{v} = {\sum\limits_{i = 1}^{n}\quad {\frac{t_{i}}{T}ɛ_{i}^{v}}}};} & (2)\end{matrix}$

where ∈_(e) ^(ν) is the resultant dielectric constant of the laminatedmaterials at a dc bias of field E, and ∈_(i) ^(ν) is the dielectricconstant of the ith layer at dc bias of field E. Then:

 ∈_(i) ^(ν)=∈_(i) ⁰(1−b _(i) E);  (3)

where b_(i) is the tunability of ith material, which is defined as:$\begin{matrix}{{b_{i} = {- \frac{\frac{ɛ_{i}^{v}}{E}}{ɛ_{i}^{0}}}};} & (4)\end{matrix}$

Therefore, the tunability of the laminated material is: $\begin{matrix}\begin{matrix}{b_{e} = \quad {- \frac{\frac{ɛ_{e}^{v}}{E}}{ɛ_{e}^{0}}}} \\{= \quad \frac{\sum\limits_{i = 1}^{n}\quad {\frac{t_{i}}{T}ɛ_{i}^{0}b_{i}}}{\sum\limits_{i = 1}^{n}\quad {\frac{t_{i}}{T}ɛ_{i}^{0}}}}\end{matrix} & (5)\end{matrix}$

In the case of all layers having the same tunability but with differentdielectric constants, equation (5) will be

b _(e) =b ₁ ; I=1,2, . . . , n  (6)

Equation (6) indicates that the laminated material has the sametunability as the individual tunability of each layer.

Now a two-layer case is considered. Layer 1 is a tunable material withhigh dielectric constant. Layer 2 is a non-tunable material with lowdielectric constant. If t₁≅t₂ or t_(l) is not much smaller than t₂, wecan get from equation (5) that

b _(e) =b ₁;  (7)

Equation (7) indicates the laminated material tunability is the same asthe tunable layer.

This invention provides a multi-layered structures of high dielectricconstant, low loss, and high tunability materials laminated with lowdielectric materials, which may be tunable or non-tunable. The inventionis not limited only to obtaining low dielectric constant materials. Anydielectric constant bounded by the dielectric constants associated withthe individual layers can be achieved with this method.

The method of laminating different layers can be simply mechanical,co-firing, or thick film and/or thin film processing. In these methods,the properties the individual layers should be the same or close to theproperties of the corresponding layers in the final laminated structure.

Accordingly, in the present invention, a laminated structure materialcan realized by alternating two or more different dielectric constantmaterials using either physical or chemical processing. The dielectricconstant can be tailored by choosing both proper materials and layerthickness with little or no loss of tunability or degradation ofdielectric loss.

An advantage of the present invention is that a certain overalldielectric constants can be easily tailored by laminating highdielectric constant material(s) with low dielectric constantmaterial(s). The resultant dielectric constant of the laminatedmaterial(s) can range from several to tens, even to hundreds ifnecessary, since the high dielectric material(s) may beBa_(1−x)Sr_(x)TiO₃ (BSTO), where x can vary between zero and one, andBSTO composites, with dielectric constants that vary from about 100 tothousands, and low dielectric constant material(s) such as air (∈=1),and/or other dielectric materials such as alumina (∈=9-10), Mica(∈=4.2), and Ba_(1−x)Sr_(x)TiO₃-based materials. While the presentinvention has been disclosed in terms of its presently preferredembodiments, it will be understood by those in the art that variousmodifications of the disclosed embodiments can be made without departingfrom the spirit and scope of this invention, which is defined by thefollowing claims.

What is claimed is:
 1. A tunable dielectric structure comprising: afirst layer of dielectric material, having a dielectric constant greaterthan 100; a second layer of dielectric material, having a dielectricconstant less than 30, positioned adjacent to said first layer ofdielectric material; first and second electrodes for applying a firstcontrollable voltage across said first layer of dielectric material,thereby controlling the dielectric constant of said first layer ofdielectric material, wherein one of said first and second electrodes ispositioned between said first and second layers of dielectric material;and means for applying a second controllable voltage across said secondlayer of dielectric material, thereby controlling the dielectricconstant of said second layer of dielectric material.
 2. A tunabledielectric structure as recited in claim 1, wherein said first andsecond layers of said dielectric material each have a respectivethickness less than about one tenth of the wavelength of a radiofrequency signal to be transmitted through the tunable dielectricstructure.
 3. A tunable dielectric structure as recited in claim 1,further comprising: a plurality of additional layers of dielectricmaterial positioned in parallel with said first and second layers ofdielectric material, and at least selected ones of said additionallayers of dielectric material respectively having a tunable dielectricconstant.
 4. A tunable dielectric structure as recited in claim 3,wherein said first, second and additional layers of said dielectricmaterial are assembled into a plurality of subassemblies.
 5. A tunabledielectric structure as recited in claim 1, wherein said first layer ofdielectric material has a loss tangent of less that 0.01.
 6. A tunabledielectric structure as recited in claim 1, wherein said second layer ofdielectric material comprises a BA_(1−x)Sr_(x)TiO₃ composite where xranges from zero to one.
 7. A tunable dielectric structure as recited inclaim 1, wherein said first and second layers of said dielectricmaterial are rectangular slabs lying in planes that are orientedparallel to a direction of propagation of a radio frequency signalthrough the tunable dielectric structure.
 8. A tunable dielectricstructure as recited in claim 1, wherein said first and second layers ofsaid dielectric material comprise one of the group of bulk, tape, thickfilm and thin film layers.
 9. A microstrip line comprising: a firstlayer of dielectric material; a second layer of dielectric materialpositioned adjacent to said first layer of dielectric material, saidsecond layer of dielectric material having a dielectric constant that isless than a dielectric constant of said first layer of dielectricmaterial; first and second electrodes for applying a first controllablevoltage across said first layer of dielectric material, therebycontrolling the dielectric constant of said first dielectric material,wherein one of said first and second electrodes is positioned betweensaid first and second layers of dielectric material; a ground planepositioned adjacent to a first edge of each of said first and secondlayers; and a microstrip positioned adjacent to a second edge of each ofsaid first and second layers.
 10. A microstrip line as recited in claim9, further comprising: a plurality of additional layers of dielectricmaterial positioned in parallel with said first and second layers ofdielectric material, and at least selected ones of said additionallayers of dielectric material respectively having a tunable dielectricconstant.
 11. A microstrip line as recited in claim 10, wherein saidfirst, second and additional layers of said dielectric material areassembled into a plurality of subassemblies.
 12. A microstrip line asrecited in claim 9, wherein said first layer of dielectric material hasdielectric constant greater than about 100 and a loss tangent of lessthan about 0.01.
 13. A microstrip line as recited in claim 9, whereinsaid second layer of dielectric material comprises one of: aBa_(1−x)Sr_(x)TiO₃ composite where x ranges from zero to one, alumina,mica, and air.
 14. A microstrip line as recited in claim 9, wherein saidfirst and second layers of said dielectric material comprise one of thegroup of bulk, tape, thick film and thin film layers.
 15. A microstripline as recited in claim 9, wherein said first and second layers of saiddielectric material each have a respective thickness less than about onetenth of the wavelength of a radio frequency signal to be transmittedthrough the device.
 16. A microstrip line as recited in claim 9, furthercomprising: means for applying a second controllable voltage across saidsecond dielectric material, thereby controlling the dielectric constantof said second dielectric material.
 17. A tunable dielectric structurecomprising: a first layer of dielectric material, having a dielectricconstant greater than 100; a second layer of dielectric material, havinga dielectric constant less than 30, positioned adjacent to said firstlayer of dielectric material; a third layer of dielectric material,having a dielectric constant greater than 100, positioned adjacent tosaid second layer of dielectric material; means for applying a firstcontrollable voltage across said first layer of dielectric material,thereby controlling the dielectric constant of said first layer ofdielectric material; and means for applying a second controllablevoltage across said third layer of dielectric material, therebycontrolling the dielectric constant of said second layer of dielectricmaterial; wherein the first, second and third layers of dielectricmaterial each have a thickness of less than one tenth of a wavelength ofa signal to be transmitted through the structure.
 18. A tunabledielectric structure as recited in claim 17, wherein said first, secondand third layers of dielectric material each have a respective thicknessless than about one tenth of the wavelength of a radio frequency signalto be transmitted through the tunable dielectric structure.
 19. Atunable dielectric structure as recited in claim 17, further comprising:means for applying a third controllable voltage across said seconddielectric material, thereby controlling the dielectric constant of saidsecond dielectric material.